Check if SSL key and certificate files exist

[toXel]

It's a first attempt. Please suggest changes.
This fixes #416.

[xPaw]

I would prefer two separate messages for each of the files. And do a negative check, then you dont need an else.

[xPaw]

Just wondering, does it explode if the file exists but it's not valid? If so might want to add an extra check for that.

[toXel]

Good idea. I'll look into this.

[astorije]

@xPaw, in #416, @toXel asked if we preferred an if condition or a try/catch block. Do you have an opinion on this? I don't really have one myself.

[toXel]

@xPaw yes it crashes if the files are not valid:

_tls_common.js:68
      c.context.setCert(options.cert);
                ^

Error: error:0906D06C:PEM routines:PEM_read_bio:no start line

I could add a try/catch block around

server = server.createServer({
    key: fs.readFileSync(keyPath),
    cert: fs.readFileSync(certPath)
}, app).listen(config.port, config.host);

to catch it and stop the server with a better message.

But then if the server couldn't be created because of some other error, it will give the user a wrong error message...

[xPaw]

Fine by me, let it crash on invalid certs, unless node has a function to validate them (which I doubt)

[toXel]

@xPaw Nope I haven't found a function for it. There's a pem npm module with a readCertificateInfo function. But I think this will also crash on invalid files.

[astorije]

Can you change these from let to const please?

[astorije]

After addressing my comment, can you squash your PR as well please? :)

[toXel]

Done :)

[astorije]

Great job @toXel, thanks!